Please review: fix page fault panic in lacp_req

[Alan Somers]

If you do an "ifconfig -am" in one thread while doing an "ifconfig
lagg0 destroy" in another thread, at least two panics may result. One
is in lacp_req(), caused by NULL == lsc.  I opened kern/189003 to
describe it.  I'm still working on the other panic or panics.

Full explanation and patch are at
http://www.freebsd.org/cgi/query-pr.cgi?pr=189003&cat= .  Can somebody
please review the patch?

-Alan